Laser output control device for an optical information recording and reproducing apparatus

Abstract: In a laser output control device for use in an optical information recording and reproducing apparatus which uses output light from a semiconductor laser device (1) to record and reproduce information onto and from an optical information recording medium (9), a light receiving element (19) is provided on the peripheral portion of a laser light emitting window (22) formed in a housing (21) of the semiconductor laser device (1). The peripheral component (5) of forward radiated light of the laser light output from the semiconductor laser device (1) is received by the light receiving element (19), so as to control the output power of the semiconductor laser device (1) in accordance with output of the light receiving element corresponding to the received light.

Description: The present invention generally relates to an optical information recording and reproducing apparatus having a laser output control device that records and reproduces information onto and from an optical information recording medium, and it more particularly relates to a device for controlling output power of a semiconductor laser. Semiconductor lasers provide light outputs that vary with changes in temperature and in luminous energy. On the other hand, optical information recording and reproducing apparatus provides automatic power control (hereinafter referred to as APC) of laser light utilized therein, because they require constant laser light output for recording and reproducing information in an uniform condition. Examples of the prior techniques relating to such automatic control of laser light output are disclosed in Japanese Patent Laid-open Publication Nos. SHO 50-23902 and 51-29821, Japanese Patent Publication Nos. HEI Nos. 2-47015 and 5-6260, and Japanese Patent Laid-open Publication No. HEI 4-330646. For example, in an optical information recording and reproducing apparatus as shown in Fig. 4, the peripheral light component 5 of forward radiated light output from a semiconductor laser chip 1 is eliminated via a collimating lens 3, so that only the non-peripheral light component 2. i.e., light component in and around the center of the forward radiated light enters the collimating lens 3 to be converted into a collimated light beam and then enters a beam splitter 4. The non-peripheral light component 2 is divided into transmitted light 6 and reflected light 7 by the beam splitter 4. The transmitted light 6 is converged by a converging lens 8 and directed onto an information recording medium 9 as a very minute laser light spot 10 to be utilized for effecting information recording or reproducing as needed. The reflected light 7 from the beam splitter 4 enters a light detection element 15. Reflected light from the information recording medium 9 goes back the same path that the transmitted light 6 took to reach the recording medium 9. The reflected light is converted via the converging lens 8 into a collimated beam, which is then directed to the beam splitter 4. Light transmitted through the beam splitter 4 is then directed onto the semiconductor chip 1 as return light, while light 11 reflected by the beam splitter 4 is received by a light detection element 13 via light receiving lenses 12. Thus, information signal, focus controlling signal and tracking controlling signal are derived from the light detection element 13. The APC executes output control to maintain the laser light output at a constant value by means of a loop circuit configured such that a part of the light output from the semiconductor laser is received by the light detection element 14 or 15, the light output from the element 14 or 15 is amplified by an output control circuit and fed back to the semiconductor laser chip. To this end, the light detection element 14 is designed to receive rearward radiated light 16 from the semiconductor laser chip 1 so as to provide the light output 17, and the light detection element 15 is designed to receive the reflected light 7 from the beam splitter 4 to provide the light output 18. In the illustrated embodiment, the rearward radiated light 16 output from the semiconductor laser chip 1 is derived via the light detection element 14 that is mounted within a semiconductor laser housing (not shown). However, this prior arrangement presents a problem that, because the forward and rearward radiated lights output from the semiconductor laser chip 1 are not in a complete correlation with each other, it can not maintain the laser light output at a constant value and hence can not provide a satisfactory APC operation. Further, because the light detection element 15 receives the reflected light 7 from the beam splitter 4, the element 15 greatly depends on the characteristics of the beam splitter 4. Thus, if, for example, a polarizing beam splitter is employed as the beam splitter 4 to reduce the transmitted light loss in the splitter and improve the optical efficiency of the optical system (usually, the beam splitter is combined with an 1/4 wave plate so as to constitute a light isolator), the reflected light 7 will not exist, so that no output can not be obtained from the light detection element 15. Furthermore, the light detection element 15 becomes large in size because it is designed to receive the reflected light 7 directly from the beam splitter 4, and thus the interelectrode capacity of the element 15 is increased, with the result that the response speed is objectionably decreased. This problem may be overcome such as by converging the light incident on the light detection element 15 so as to reduce the size of the element 15. To do this, it is necessary to newly provide a light converging means, but such a new provision of the converging means is not compatible with the current demand for reducing the size and cost of the apparatus. JP-A-04 330 646, which is shown in Fig. 1 discloses a laser output control device for use in an optical information recording and reproducing apparatus which comprises a semiconductor laser device and an optical system for using the laser light generated by the semiconductor laser device to record and reproduce information onto and from an optical information recording medium, said laser output control device comprises a light detection element for receiving a peripheral light of forward radiated light of laser light generated by the semiconductor laser device and outputting an electric signal corresponding to the received peripheral light, and a control section for controlling output power of the laser light generated by the semiconductor device, in accordance with output of the light detection element. US 5 191 204 A, on which the preamble of claim 1 is based, further discloses a housing. According to the invention, a laser light transmitting window is provided in a housing of the semiconductor laser device and the light receiving element is disposed outside said housing on the periphery of the transparent plate covering the window. The non-peripheral light component in and around the center of the forward radiated light from the semiconductor laser device passes through the laser light transmitting window of a housing containing the laser device toward an optical information recording medium. On the other hand, the peripheral light component of the forward radiated light enters the light detection element disposed on the periphery of the laser light transmitting window, from which electrical output is obtained which corresponds to the forward radiated light from the semiconductor laser device. This electrical output is provided to an output control circuit of the semiconductor laser device, which controls the output power of the laser device. The described optical system can control the output power of the semiconductor laser device corresponding to the forward radiated light output from the laser device, and thus the invention can achieve advantages that the output power control provided by the invention is far more effective than the correlation-lacking feedback control provided by the prior technique using rearward radiated light, and any sort of beam splitter can be employed. Further, because a smaller-size light detection element can be utilized, the response speed of the element never decreases and thus it is not necessary to provide any means for converging the light incident on the element in order to prevent such a response speed decrease, with the result that the output power control and APC response characteristics of the laser light can be highly enhanced, limitations on the device design can be eliminated, and it is also allowed to substantially reduce the size and cost of the device. Now, the preferred embodiment of the present invention will be described in detail below with reference to the accompanying drawings. In the accompanying drawings: Fig. 1 is a schematic side elevational view of a prior art optical system; Fig. 2 is a schematic side elevational view illustrating the structure of the semiconductor laser section according to the invention; Fig. 3A is a schematic top plan view illustrating one example of the structure and arrangement of a light-detection element; Fig. 3B is a schematic top plan view illustrating another example of the structure and arrangement of the light-detection element; Fig. 3C is a schematic top plan view illustrating still another example of the structure and arrangement of the light-detection element: Fig. 3D is a schematic top plan view illustrating still another example of the structure and arrangement of the light-detection element; and Fig. 4 is a schematic side elevational view illustrating the structure of an optical system employed in a prior art optical information recording and reproduction device. Referring to Fig. 1 in which is shown a prior art optical system, same reference characters as in Fig. 4 denote same components as in the figure. To first describe the optical configuration of the optical system, the non-peripheral light component 2 in and around the center of forward radiated light output from a semiconductor laser chip 1 passes through a central aperture formed in a light detection element 19 which is, for example, in the shape of a doughnut or ring, and then enters a collimating lens 3, from which the non-peripheral light component enters a beam splitter 4 in a collimated light beam. The beam splitter 4 may be a polarizing beam splitter through which all the incident light is allowed to pass. After that, in a similar manner to the above-mentioned in relation to Fig. 4, information signal, focus control signal and tracking control signal are derived from a light receiving element 13. Fig. 2 shows in detail an embodiment of the invention incorporated in semiconductor laser section that accommodates the semiconductor laser chip 1. The semiconductor laser chip 1 is mounted within a housing 21, and the interior of the housing 21 is maintained airtight. The housing 21 has a laser light transmitting window 22 which is covered with a transparent flat plate 23, and, for example, the doughnut-shaped or ring-shaped light receiving element 19 is preferably adhered to the transparent flat plate 19. The doughnut-shaped light detection element 19 is adhered to the side of the transparent flat plate 23; opposite to the laser chip 1. The novel structural feature of the present invention is that the light detection element 19 is disposed on the periphery of the laser light transmitting plate 22 covering the window of the housing 21 and the output from the element 19 is fed back to a semiconductor laser output control circuit 20. Namely, in Fig. 2, the non-peripheral light component 2 in and around the center of the forward radiated light output from the semiconductor laser chip 1 enters the collimating lens 3 through the central aperture of the doughnut-shaped light detection element 19, and the peripheral light component of the radiated light enters the light detection element 19. In this case, the percentage with which the peripheral light component 2 enters the collimating lens 3 depends on the NA (numerical aperture) of the collimating lens 3. If, for instance, the NA of the collimating lens 3 is chosen to be 0.3 (NA = 0.3) as is often the case with the conventional optical information recording and reproducing systems, the light component within a 35° solid radiation angle of the forward radiated light output from the semiconductor laser chip 1 can enter the collimating lens 3. This accounts for about 60% of the forward radiated light, and the remaining 40% is contained in the peripheral light 5 and is received by the light detection element 19. The light detection element 19 may be of any shape as long as it can receive the whole or part of the peripheral light 5 of the forward radiated light. For instance, the light receiving element 19 may comprise a doughnut-shaped element 19-1 as shown in item Fig. 3A or a square or rectangular element 19-2 having a central aperture formed therein as shown in Fig. 3B, or it may comprise a predetermined plurality of separate elements 19-3 or 19-4 as shown in Fig. 3C or 3D. In the described optical system, light output corresponding to the peripheral light component of forward radiated light from a semiconductor chip is obtained by a light detection element and fed back to and controlled by a semiconductor laser output control circuit. Because of this feature, the described optical system can achieve superior advantageous results that the output power control provided by the system is far more effective than the correlation-lacking feedback control provided by the prior technique using rearward radiated light, and any sort of beam splitter can be employed. Further, because a smaller-size light detection element can be utilized, the response speed of the element never decreases and thus it is not necessary to provide any means for converging the light incident on the light detection element in order to prevent such a response speed decrease, with the result that the output power control and APC response characteristics of the laser light can be highly enhanced, limitations on the device design can be eliminated, and it is also allowed to substantially reduce the size and cost of the device.

Claims: Optical information recording and reproducing apparatus comprising: a semiconductor laser device (1) to be used as a light source for information recording and reproduction, light detection means (19) receiving a peripheral light of forward radiated light of the laser light generated by said laser device (1), said light detection means adapted to output an electrical signal representing an amount of the received peripheral light, control means (20) for, in accordance with the output electrical signal of said light detection means (19), automatically controlling output power of the laser light generated by said laser device (1) to be maintained at a predetermined value, and an optical system (3, 4, 8, 12, 13) for allowing the laser light generated by said laser device (1) to be irradiated onto an optical information recording medium (9), said optical system being arranged in such a manner that laser light reflected from said optical information recording medium (9) is fed back along the optical path and branched toward a reading light detection element (13) via splitter means (4) a housing (21) for accomodating said semiconductor laser device (1) characterized in that said housing (21) has a laser light transmitting window (22) for allowing passage therethrough a laser light generated by the semiconductor laser device (1), said laser light transmitting window (22) being covered by a transparent plate (23) and said light detection means (19) are disposed outside said housing (21) on the periphery of said transparent plate (23). Optical information recording and reproducing apparatus as defined in claim 1 wherein said light detection means (19;19-1;19-2) is disposed in a ring-shaped configuration on the periphery of the transparent plate. Optical information recording and reproducing apparatus as defined in claim 1 wherein said light detection means (19;19-3;19-4) comprises a plurality of segments disposed on the periphery of the transparent plate.

Inventors: NAKAMURA EIICHI, NODA KAZUO, YAMAZAKI KOICHI, NAKAMURA, EIICHI, NODA, KAZUO, YAMAZAKI, KOICHI
Applicants: NIPPON CONLUX CO LTD, NIPPON CONLUX CO., LTD.
Application year: 1994
Main classification: G11B7
IPC codes: G11B7